Metal parts often fail their intended use due to wear, which causes them to lose dimension and functionality. “Hardfacing” is a technique which involves applying a layer of hard material to a substrate for the purpose of increasing the substrate's wear and corrosion resistance. The use of this technique has increased over the years as industry has come to recognize that substrates of softer, lower cost material can be hardfaced to have the same wear and corrosion resistance characteristics as more expensive substrates of a harder material. Hardfacing may be applied to a new part during production to increase its wear resistance, or it may be used to restore a worn-down surface. Hardfacing extends the service life of the workpiece and can save machine down time and production costs.
Hardfacing involves the deposition of a hard layer by welding or thermal spraying. Conventional weld hardfacing is accomplished by one of several welding techniques, including oxyfuel welding (OFW), gas tungsten arc welding (GTAW or TIG), hot wire GTAW, gas metal arc welding (GMAW), hot wire GMAW, shielded metal arc welding (SMAW), submerged arc welding (SAW), and flux-cored arc welding (FCAW). Plasma transferred arc (PTA) hardfacing and laser beam hardfacing can also be used. In general, a welding wire is deposited over the substrate surface to produce a weld deposit that is more wear resistant than the underlying substrate.
Hardfacing alloys are designed to provide improved wear resistance for a specific wear factor or a combination of wear factors. Abrasion performance of the deposited alloy is directly related to the amount of carbide forming metals, such as chromium, molybdenum, tungsten, vanadium, and iron, in combination with carbon. Wear resisting carbides are formed when one of these metals reacts with carbon, and the balance of the carbon remains in solution to form a semi-austenitic matrix in which the hard, wear resistant carbides are evenly distributed. As the ratio of the wear resistant carbides to the alloy matrix increases, abrasion resistance increases while at the same time its impact resistance decreases.
One type of hardfacing materials are alloys known as “chromium carbides.” Their high abrasive resistance is derived from the presence in the microstructure of primary chromium carbides (M7C3) of the eutectic and/or hypereutectic type in a soft tough matrix. Because these alloys contain large amounts of chromium carbide, they are particularly good for severe abrasion resistance applications. However, chromium carbide alloys having a hardness on the Rockwell “C” hardness scale (“HRC”) greater than 62 HRC have been difficult to achieve consistently. While the primary chromium carbides formed may themselves exhibit hardness values of around 63-65 HRC, the eutectic matrix, which is the material surrounding the primary carbides, exhibits hardness values in the range of 50 to 58 HRC. This makes it difficult to achieve a consistent average hardness measurement greater than 62 HRC in a first layer deposit of such chromium carbide alloys. While weld deposits with low levels of boron or niobium or both in the eutectic matrix may exhibit higher hardness measurements at points in the eutectic matrix, the hardness tends to be inconsistent across the weld bead.
While conventional chromium carbide alloys provide good wear resistance, the weld deposits produced from chromium carbide welding wires can produce a cross-checking pattern in the hard weld deposit surface. Unless hardfacing deposits with these types of microstructures cross-check to relieve the stresses, under-bead cracks may form and material will spall from the surface. While cross-check cracking that is uniformly distributed is desirable as it indicates a consistent microstructure, longitudinal cracking is detrimental as it contributes to the likelihood of under-bead cracking. Cross-check cracking may also trap abrasive material within the space formed by the cracks and allow the abrasive material to absorb some of the wear, adding to the total abrasive resistance of the deposit.
In view of the present state of hardfacing technology, it would be highly desirable and advantageous to provide an electrode for depositing a hardfacing alloy composition having a primary carbide eutectic microstructure of high abrasive resistance for use on the surface of metal components that are subjected to high thermal and mechanical stresses and that can consistently achieve a hardness value of at least about 65 HRC in the first layer of weld deposit. Such hardfacing alloys can produce the same amount of wear resistance using a thinner layer of the hardfacing alloy, or to extend further the service life of equipment by using the same amount of the hardfacing alloy.